Recently, in order to save current in a transmit chain, the mobile phone manufacturers have been moving towards “envelope tracking”, which is a technique where the power amplifier is supplied through a fast DC/DC converter whose output voltage is varying over time as a function of the amplitude modulation. The concept of envelope tracking is to operate as close as possible to saturation during the modulation peaks and to lower the voltage when the instantaneous amplitude signal is low, thereby boosting the power amplifier efficiency.
However, there are significant challenges in this concept. In fact, the gain of the power amplifier is affected by the DC/DC voltage. Thus, if one simply tries to follow the peaks of the signal with the DC/DC converter, the gain variation will result in a distortion of the modulation.
Furthermore, AM/PM phenomena may take place, which will also impair the modulation quality, therefore resulting in spurious emissions (unwanted energy in neighboring channels) or an error vector magnitude (EVM) degradation.
In conventional systems there are two ways to minimize the unwanted phenomena highlighted above. One conventional approach is to choose the trajectory of the DC/DC control voltage accurately so that the power amplifier gain stays constant. It has to be noted, however, that as the signal level increases and the power amplifier approaches saturation, its instantaneous gain diminishes. In particular, the intention of envelope tracking is to increase the DC/DC voltage when the amplitude signal goes through a peak. Here, increasing the DC/DC voltage generally leads to a gain increase. By combining these two effects, a cancellation can be obtained; hence limiting the unwanted distortion of the signal. For this concept, the AM/PM phenomena introduced by the power amplifier should be negligible.
Another conventional approach is to compensate both AM/AM and AM/PM distortions by adequately predistorting the input wave into the power amplifier. This can be accomplished with an analog real-time closed loop architecture or using some fixed predistortion. The predistortion based on closed loop architectures typically requires extremely wide bandwidth in order to not create excess noise at a duplexer offset. When using the predistortion without the closed loop architecture, it is typically required that the characteristic of the power amplifier is known with good detail.
The first conventional approach of the envelope tracking relies heavily on the knowledge of the so-called “isogain” contours, which have to be individually calibrated on each phone. However, also the second conventional approach of the predistortion requires the knowledge of the AM/AM and the AM/PM curves as a function of the instantaneous DC/DC voltage.
A disadvantage of the first conventional approach is that calibrating the isogain contours is a long task, which prolongs the calibration time in the factory. Also, the calibrated isogain contours typically have to be stored in a random-access memory (RAM) and they are characterized in that they are fixed. This results in the fact that if the power amplifier characteristic is not perfectly stable over different conditions (e.g. aging, temperature, load, etc.), the matching of the gain loss because of a proximity to saturation and the gain expansion because of an increased DC/DC voltage can no longer be achieved, therefore leading to a spectrum worsening.
A disadvantage of the second conventional approach is that the AM/AM and AM/PM predistortion also requires a significant individual calibration. Furthermore, its adequateness is typically not always guaranteed under all circumstances.
Therefore, conventional systems are disadvantageous in that they are rather inflexible and in that a time-consuming calibration task in the factory is required.